Our goal is to generate the first high-resolution maps of mammalian recombination activity for an entire chromosome as a representative of the genome as a whole, including the variation in hotspot location and activity within and between species, and how this differs in F1 animals from reciprocal crosses and between male and female meiosis. For this we will map recombinants arising in 3000 meioses from each of six genetic crosses at under 50-Kb resolution, enabling us to distinguish individual hotspots and detect activities as low as 0.03 cM/hotspot. These data will go far in helping us understand mammalian recombination as a biological process as well as relationships between parameters of recombination and other aspects of chromosome dynamics. Aim 5a. we will map the location of every crossover on Chromosome 8 arising in six genetic crosses, 3000 offspring each for a total of 18,000 meioses, at under 50 kb resolution. Four of the crosses will be female F1 hybrids created in a daisy-chain design (C57BL/6J x PWD/PhJ x WSB/EiJ x CAST/EiJ x C57BL/6J) and then backcrossed to C57BL/6J. F1 male hybrids from crosses of C57BL/6J x PWD/PhJ and WSB/EiJ x CAST/EiJ will also be backcrossed to C57BL/6J. Whenever possible, equal numbers of meioses will be derived from F1 animals created by reciprocal crosses of the original parents (AxB and BXA). This design is optimal for gathering data on genetic variability, sex specificity, and possible imprinting effects in these crosses. Aim 5b. using the data of aim 5a, we will analyze the strain, sex and cross direction specificity of the location, distribution and activity of the recombination hotspots along the length of Chromosome 8. For understanding evolutionary processes, we will also provide a consensus map of Chromosome 8 hotspots across subspecies. Aim 5c. we will make the 18,000 backcross DMA samples available to the scientific community via a repository, and the 3000 SNP assays that will be newly developed on Chromosome 8 will be posted on our web site and made available to others for QTL and gene mapping studies.